1. Field of the Invention
The present invention relates to a transformer, and more particularly, a transformer having the number of turns being adjustable according to an outputted voltage.
2. Description of the Prior Art
A transformer may transform an alternating current (AC) or a direct current (DC) power to a DC power of a specific range, so it is widely adopted in the field of electrical equipment and power supply. Most commonly used transformers include flyback transformers (FBT), forward transformers and push-pull transformers. FIG. 1 illustrates a transformer 100 according to the prior art. The transformer 100 includes a winding Ta having a number of turns Na and a winding Tb (disposed at a secondary side) having a number of turns Nb so as to have a winding ratio as Na/Nb. The winding Ta is disposed at a primary side and coupled to an input power supply Vi. The winding Tb is disposed at a secondary side and coupled to an output voltage terminal Vo. The winding Ta is controlled by a pulse-width modulation (PWM) signal Vp outputted from a PWM unit 110. The PWM signal Vp has a duty cycle, and an ideal value of the duty cycle is 50%. However, the duty cycle varies according to the winding ratio Na/Nb and an output voltage at the output voltage terminal Vo. If the winding ratio remains unchanged, the PWM unit 110 is controlled by a feedback circuitry to increase the duty cycle when the output voltage increases, and the duty cycle is decreased when the output voltage decreases.
According to the prior art, when the output voltage varies, the duty cycle is difficult to keep stable. For example, when the input power supply Vi supplies a 90 Volt (V) DC power for the output voltage terminal Vo to output a 5V DC power, the winding ratio Na/Nb may be designed as 48/3 for the duty cycle to be 47.3%, close to the ideal value 50%. However, when winding ratio remains at 48/3, and the output voltage at the output voltage terminal Vo is changed to a higher voltage such as 20V, the duty cycle may increase to be 76.7% so that the duty cycle is too high, unwanted oscillations, overheat and incorrect operations are thus easier to occur, and this is harmful for the reliability of the circuit. For another example, when attempting to receive a 90V AC power from input power supply Vi and output a 20V DC power at the output voltage terminal Vo, the winding ratio Na/Nb maybe selected as 48/10 so as to have a 49.7% duty cycle which is nearly ideal. However, if the winding ratio remains 48/10, when output voltage at the output voltage terminal Vo changes to be 5V, the duty cycle may decrease to be 21.2% and be too low so that the efficiency of the transformer is lowered.
Therefore, when using the transformer 100 of the prior art, when adjusting the output voltage according to various applications, the duty cycle easily gets too high or too low and is difficult to keep stable.